Many industrial applications have increasing requirements for superior wear and corrosion resistant surfaces. This is particularly true in the case of abrasive wear in a corrosive environment. Solid, sintered tungsten carbide-cobalt or similar solid, sintered materials with very high hardnesses have been used successfully as solid components or as inserts in some applications, but in many situations their use is impractical because of their lack of structural toughness, high cost or fabrication difficulties. In such situations the only practical solution is an overlay or coating on a metallic substrate. Weld-deposited hard facing compositions are the most common type of overlays that are used in abrasive environments. A variety of welding techniques are used to deposit these materials, but oxygen-acetylene welding or flame spraying using wire, rod or powder is probably the most widely used. A common technique is to deposit the materials in one or more welding passes and then subsequently remelt or fuse the deposit with the same or a different welding torch, or, in some cases, by furnacing. All of the welding techniques involve some melting of the substrate surface and hence dilution of the deposit with substrate metal.
The most commonly used hard facing compositions are tabulated by the American Society for Metals (ASM) in Metal Process, Vol. 112, No. 6, Nov. 1977, p. 49. The ASM class 5 hard facing materials containing 75 to 96 weight percent (all compositions hereinafter are given in weight percent) tungsten carbide as unmelted crystals or grains in a cobalt-base alloy are the most wear resistant, but are generally very brittle with poor mechanical shock resistance and are difficult to apply without cracking, excessive oxidation, etc. The next most abrasion resistant hard facing materials are the ASM class 4 alloys consisting of Ni (nickel) and Co (cobalt) base alloys. The Co-base alloys of sub-class 4A contain W (tungsten) and C (carbon) in solution and hence may form a limited volume fraction of tungsten carbide by precipitation during cooling. These overlays, as well as those of class 5, are usually applied using a flux covering, inert gas shrouding or some other means to minimize oxidation during deposition and subsequent fusing. The Ni-base alloys of sub-class 4B are known as "self-fluxing" alloys and contain B (boron) and Si (silicon) to form their own flux. During initial deposition or post deposition remelting, most of these fluxing elements combine with the metal oxides and float to the surface of the deposit forming a slag. Although the self-fluxing alloys are normally used by themselves, they are occasionally combined in a mixture with unmelted tungsten carbide grains to improve the toughness of the class 5 hard facing alloys. A recent example is that of Patel U.S. Pat. Nos. 4,013,453 and 4,075,371, in which a nickel-base alloy containing 0.5 to 5 B and 0.5 to 6 Si is mixed with a WC-Ni agglomerate and flame sprayed. Patel intentionally avoids significant reaction between the WC particles and the metallic matrix to preserve the initial powder size and distribution of WC grains.
The most commonly used self-fluxing alloys contain up to 3.5 B and up to 4.5 Si. Ni-base alloys with up to 6 B and Si are taught by Tour in U.S. Pat. No. 2,875,043 for the special case of the alloys of his invention which contain, in addition, 3 to 10 Mo (molybdenum) and 3 to 8 Cu (copper). The latter presumably increases the fluidity of these otherwise conventionally used self-fluxing alloys. He does not teach the use of these alloys with a carbide hard phase or for any other purpose than oxygen-acetylene spray welding including remelting.
Quaas in U.S. Pat. No. 3,341,337 teaches the admixture of boric acid to Ni, Co or Fe (iron) base flame spray powder (oxygen-acetylene hard facing), with or without an admixture of tungsten carbide grains, to prevent oxidation during deposition. Most of the boric acid is intentionally lost by vaporization or as slag. It is not intended to react with or form part of the deposit.
Schrewelius in U.S. Pat. No. 3,025,182 teaches the production of a nonporous corrosion resistant coating by oxygen acetylene flame spraying of a mixture of a metal and 2 to 40 B or boron containing compound. He specifically teaches that both components have a melting point greater than 1300.degree. C. and that most of the boron is volatized as boric acid as a result of its fluxing action. This is particularly important, he teaches, since B is often detrimental to the final deposits.
Noguchi, et al. in Nippon Tungsten Review, Vol. 1 (Sept. 1974) pages 54 to 58 have described a variant of the normal WC (tungsten carbide) self-fluxing mixture for oxygen-acetylene spraying, namely a prealloyed powder containing greater than 25 WC, 8 to 12 Cr (chromium), 2 to 4 Fe, 5 to 6 (B+ Si+ C), balance Ni. The specific amount of B, Si and C is unspecified and presumably could be zero for any one. Although dense coatings are reported with better wear resistance than self-fluxing alloys blended or mixed with WC, the hardness of the coatings (less than 1000 DPH.sub.300) (all hardness values hereinafter given in kg/mm.sup.2 as measured on the diamond pyramid hardness scale with a 300 g load), is still lower than conventional detonation gun (d-gun) WC-C. coatings.
In spite of all the effort that has been expended on weld-deposited hard facing overlays or coatings, none have approached the sintered tungsten carbides in wear resistance. Nonetheless they are widely used where the sintered products cannot be applied.
Another type of material, plasma and d-gun coatings, has been used for many years for wear-resistant applications requiring only relatively thin coatings, usually 0.002 to 0.020 inches thick, and moderate hardness, up to about 1200 DPH.sub.300 but usually less than 1000 DPH.sub.300. One of the most useful classes of compositions for this purpose has been the family of tungsten carbide-cobalt coatings (usually with 10 to 25 wt.% cobalt). As with all as-coated plasma and detonation gun coatings, these tungsten carbide-cobalt coatings have some interconnected porosity. This porosity reduces the corrosion resistance of the coating and, to some degree, its wear resistance, particularly its abrasive wear resistance. Thus, while these coatings are used very successfully in a wide variety of adhesive wear situations, they have been used with much more limited success in severe abrasive environments.
Compositions similar to the self-fluxing hard facing alloys have been adapted for plasma deposition where they are generally used without post deposition fusion of the coating. They simply increase the fluidity of the molten powder particles as they impact on the surface being coated and potentially facilitate particle-to-particle bonding through a localized fluxing action. The result is presumably a denser, stronger coating with less interconnected porosity. Typical compositions for this kind of an application contain boron, silicon or phosphorus both to act as a reductant (fluxing agent) and to lower the melting point and increase the fluidity of the metals used as in the case of hard facing self-fluxing alloys. Boron is usually present up to about 3.5 wt.% and silicon up to about 4 wt.%. Phosphorus is used less frequently than boron or silicon. While some success has been achieved in plasma deposition in increasing the density and strength of the deposits by using these so-called self-fluxing compositions, complete sealing has not been achieved and no substantial strengthening due to the formation of new phases containing silicon or boron has been noted.
The use of the same self-fluxing alloys mixed with a tungsten carbide-cobalt powder for plasma deposition of a coating is also known in the art. As in the case of use of self-fluxing alloys by themselves, the plasma sprayed mixed coatings are not remelted or fused after deposition, and again the purpose of the self-fluxing component is to increase fluidity in an attempt to reduce porosity and to act as a localized reductant. A particularly complex plasma sprayed coating is taught by Cromwell, et al., in U.S. Pat. No. 3,936,295 that consists of a tungsten carbide-Co component (15 to 39 wt.%), nickel-aluminum (0 to 10.5 wt.%) and nickel-molybdenum (26.7 to 85 wt.%) alloys and a self-fluxing alloy (0 to 47.8 wt.%). The basis of this invention is the inclusion of the exothermically reacting Ni--Al and Ni--Mo components. Although a non-essential component of the invention, the self-fluxing alloy, when used, contains 2.75 to 4.75 B and 3.0 to 5.0 Si with these elements performing their usual self-fluxing action.
A variety of methods have been attempted in an effort to fill or infiltrate the porosity of various coatings or other porous bodies. For example, the porosity in plasma or d-gun coatings has been filled with various organic compounds (such as epoxies) to improve corrosion resistance at low temperature; however, at about 260.degree. C. or less, these materials decompose and lose their efficacy as a sealant. Moreover, these sealants can only penetrate the coating with difficulty and seldom completely seal the full thickness of the coating. They add very little, if any, mechanical strength to the coating and have been shown to add virtually nothing to the abrasion resistance of the coating.
Goetzel, et al., in U.S. Pat. Nos. 2,942,970, 2,581,252 and 2,752,666 teaches a method of producing a carbide structural body (not a coating) by first producing a very porous skeleton (30 to 70 percent porosity, higher than can be obtained by plasma or d-gun) of the carbide with a low fraction of metallic binder and then infiltrating this skeleton with a ductile, heat and corrosion resistant Ni, Co or Fe base alloy. There is no mention in the reference of the use of B, Si, or P (phosphorus) in the infiltrant and, by necessity, they must be excluded in any substantial amount to satisfy the requirement of heat resistance and high ductility. Goetzel, et al., in U.S. Pat. No. 2,612,442 teaches a corrosion resistant coating of Cr, Zr (zirconium), Al (aluminum) or Si, for the preceding, fully infiltrated carbide bodies, principally by pack cementation. In another method of coating these fully infiltrated bodies, Goetzel et al. in U.S. Pat. No. 2,899,338 teaches a coating made by first coating the fully dense, infiltrated carbide body with an undercoat or bond coat of an Fe, Co or Ni base alloy with 0-20 Mg (magnesium), 0-5 B, 0-12 P, 0-4 Si, 0-2 Mn (manganese) 0-2 C, the alloying elements not exceeding 20 percent of the total composition. This bond coat is then overcoated with a primary coating of the Ni-Cr type 4 to 10 times as thick as the undercoat, and the coated part heat treated to diffuse the bond coating into both the primary coating and the base material. B, Si and P are used primarily to lower the melting point of the bond coat to allow diffusion/infiltration of the primary coating without melting it. No reaction with the primary coating or base material is taught or the use of B, Si or P as other than a melting point depressant and there is no criticality of B, Si or P content other than enough to depress the melting point.
Breton, U.S. Pat. No. 3,743,556, uses a somewhat similar technique in that he first deposits an Fe, Ni or Co base alloy on a substrate, then a layer of "filler" material (diamond, WC, TaC, hard alloys, borides, etc.), both layers being held in place with an organic binder. On heating, the organic binder decomposes and the first layer melts and infuses the outer layer. Breton does not disclose any data characterizing his end product in terms of hardness or wear resistance. We have found that using techniques similar to those taught by Breton result in coating having a hardness less than 1000 DPH.sub.300.
It is also known in the art that porous metal structures (not coatings) can be infiltrated with a lower melting metal; e.g., Gebaurer, U.S. Pat. No. 1,342,801, who simply used a metal of lower melting point or Bourne, in U.S. Pat. No. 2,401,221, who teaches the infiltration of porous compacted iron briquettes with copper presaturated with iron to minimize reaction.